1. Field of the Invention
The present invention relates to a vacuum pump, such as a turbo-molecular pump, and more particularly to a magnetic bearing system for supporting a rotary shaft of a vacuum pump in a non-contact manner.
2. Description of the Related Art
In vacuum apparatuses, a suction or evacuation mechanism, such as a turbo-molecular pump, is used for sucking or evacuating gas from a vacuum vessel or the like to produce a vacuum therewithin. Generally, a turbo-molecular pump is equipped with a pump unit having a stator and a rotor, and designed to rotationally drive the rotor by an electrically-driven motor to suck/evacuate gas from a vacuum vessel or the like.
In most types of the vacuum pumps, such as a turbo-molecular pump, a rotary shaft rotationally holding the rotor is supported by a magnetic bearing system which comprises a position sensor and an electromagnet each installed between a pump casing and the rotary shaft. The position sensor includes a radial position sensor having a pair of sensor elements disposed opposed to each other across the rotary shaft in a radial direction of the rotary shaft. The electromagnet includes a radial-bearing electromagnet for rotatably supporting the rotor in the radial direction in a non-contact manner while being controlled based on radial position information sensed by the radial position sensor.
The rotary shaft has a lower end provided with a thrust target, and the position sensor further includes a thrust position sensor disposed opposed to the thrust target in a thrust direction of the rotary shaft. The thrust target has a rotor disc threadingly mounted thereon, and the electromagnet further includes a thrust-bearing electromagnet having a pair of electromagnet elements disposed opposed to each other across the rotor disc in the thrust direction. The thrust-bearing electromagnet is adapted to rotatably support the rotor in the thrust direction in a non-contact manner while being controlled by thrust-directional position information sensed by the thrust position sensor.
It is known to use a brushless DC motor as a motor for rotationally driving the rotor. For example, in case of using the brushless DC motor, the rotary shaft is provided with a permanent magnet of the motor, and a rotational position of the rotor is sensed to detect a position of a magnetic pole of the permanent magnet, i.e., a motor magnetic pole position. Then, an excitation pattern for use in exciting each stator coil of the motor is created based on the detected motor magnetic pole position, and the motor is controllably driven according to the excitation pattern.
A semiconductor element-based sensor such as a hall sensor, or an inductance sensor, is used as the rotational position sensor. For example, in case of using an inductance sensor, the thrust target is provided with a cutout, or a magnetic piece having a magnetic permeability different from that of a body of the thrust target, at a roatational position determined in conformity to the motor magnetic pole position, and the inductance sensor is arranged to sense an inductance change caused by the cutout or magnetic piece so as to detect the motor magnetic pole position (see, for example, Japan Patent Publication JP 2001-231238A).
As described above, the magnetic bearing system is designed to process sensor signals from various sensors so as to perform a magnetic bearing control, such as a levitation control of magnetically levitating and supporting the rotary shaft in a non-contact manner based on a magnetic levitation section, and a motor drive control based on a motor magnetic pole position detected by a rotation detection section, and provide a rotor temperature monitoring function according to a rotor-temperature detection section based on a Curie temperature of a temperature-detection magnetic segment.
In the conventional magnetic bearing system, the signal processing of the above sensor signals is performed using a plurality of signal processing circuits provided for the respective sensor signals.
FIG. 17 is an explanatory schematic diagram showing a vacuum pump equipped with a conventional magnetic bearing system, and FIG. 18 is an explanatory block diagram showing a control circuit of the conventional magnetic bearing system.
As shown in FIG. 17, the vacuum pump 6 comprises a motor-driven rotor 13 housed in a casing 10. The rotor 13 is provided with a plurality of rotary blades 14, and designed to be rotated to allow the rotary blades 14 to be rotationally moved at a high speed relative to a plurality of stationary blades 15 fixed to the casing 10 so as to suck gas from an inlet port 11 and discharge the gas from an outlet port 12 to evacuate gas molecules from a vacuum vessel (not shown) fluidically connected to the inlet port 11.
The vacuum pump 6 further includes a rotary shaft 16 coaxially fixed to the rotor 13 and adapted to be rotationally driven by a drive motor 20, such as a DC motor, so as to rotate the rotor 13. The drive motor 20 comprises a magnetic pole 21 mounted in the rotary shaft and a coil 22 fixed relative to the casing 10. The rotary shaft 16 is adapted to be supported by a radial bearing and a thrust bearing in a non-contact manner.
The radial bearing (X/Y-axial bearing) includes four radial-bearing electromagnets 31 (31a to 31d) each formed such that it is located across the rotary shaft 16, and four radial position sensors 30 (30a to 30d) each operable to sense a displacement of the rotary shaft 16 in one of four radial directions. In the levitation control, a current to be supplied to each of the radial-bearing electromagnets 31 is adjusted based on the displacement sensed by a corresponding one of the radial position sensors 30 to levitate the rotary shaft 16 at a predetermined position in the radial directions. In FIG. 17, two groups of radial bearings are disposed on vertically opposite sides of the drive motor 20, respectively.
The thrust bearing (Z-axial bearing) includes a pair of thrust-bearing electromagnets (Z-axis electromagnets) 41 disposed, respectively, on vertically opposite sides of a rotor disc 42a coaxially fixed to the rotary shaft 16, and a thrust position sensor 40 (Z-axis sensor) operable to sense a displacement of the rotary akis 16 in a thrust direction. In the levitation control, a current to be supplied to each of the thrust-bearing electromagnets 41 is adjusted based on the displacement sensed by the thrust position sensor 40 to levitate the rotary shaft 16 at a predetermined position in the thrust direction. In the vacuum pump illustrated in FIG. 17, a target member 2 is fixed to a lower end of the rotary shaft 16, and the thrust position sensor 40 (Z-axis sensor) is disposed in opposed relation to a central region of a bottom surface of the target member 2.
A rotation sensing sensor 23 is disposed in opposed relation to an outer peripheral region of the bottom surface of the target member 2. This rotation sensing sensor 23 is operable to sense a given pattern provided in the target member 2 so as to detect a phase position of the magnetic pole 21 mounted in the rotary shaft 16. In the motor drive control, the drive motor 20 is drivingly controlled based on the detected phase position of the magnetic pole.
In the control circuit of the magnetic bearing system illustrated in FIG. 18, the rotary shaft 16 is rotationally driven by the drive motor 20 which is controlled by a drive control section 102, and magnetically levitated by the radial bearing and the thrust bearing in a non-contact manner.
This control circuit of the magnetic bearing system includes a temperature detection section 60 and a rotation detection section 70. The temperature detection section 60 comprises a rotor temperature sensing sensor 60a, and rotor temperature detection means for detecting a rotor temperature based on a sensor signal output from the rotor temperature sensing sensor 60a. The rotor temperature sensing sensor 60a is operable to sense a temperature-dependent change in magnetic permeability of a temperature-detection magnetic segment mounted relative to the rotor and to send a sensor signal to the rotor temperature sensing sensor 60a, and the rotor temperature detection means 60c is operable to process the sensor signal so as to detect a rotor temperature from the permeability change and output a rotor temperature signal. The rotor temperature sensing sensor 60a is driven by carrier wave generation means 60b and is operable to output an AM modulated wave formed by modulating a carrier wave in accordance with the temperature of the temperature-detection magnetic segment.
The rotation detection section 70 comprises a rotation sensing sensor 70a, and rotation detection means 70c for detecting a rotor rotation based on a sensor signal output from the rotation sensing sensor 70a. When the motor is a brushless DC motor having a permanent magnet, the rotation sensing sensor 70a is operable to sense a position of a step, or a magnetic piece with a different magnetic permeability from that of a body of the target member 2, which is provided in the target member 2 at a position determined in conformity to a phase position of a magnetic pole of the permanent magnet of the motor and to send a sensor signal to the rotation detection means 70c. The rotation detection means 70c is operable to identify a magnetic pole position based on the sensor signal received from the rotation sensing sensor 70a and to output a signal indicative of a magnetic pole position. The rotation sensing sensor 70a is driven by carrier wave generation means 70b and operable to output an AM modulated wave formed by modulating a carrier wave with a rotation signal indicative of the sensed position.
As above, in the conventional magnetic bearing system, the signal processing of the sensor signals is performed using a plurality of signal processing circuits provided for the respective sensor signals. Thus, a size of the signal processing circuits becomes larger to cause an increase in cost.
As measures against this problem, it is contemplated to process the sensor signals by a single signal processing circuit.
In a control circuit designed to supply carrier signals to a plurality of sensors individually and asynchronously sample sensor signals, it is necessary to shorten a sampling interval of each sensor signal so as to accurately detect a moderation wave (i.e., sensed signal) from the sensor signal (i.e., moderated wave). The sampling interval can be shortened only if a carrier wave with a higher frequency is used, and a sampling operation is performed at a frequency at least two times greater than that of the carrier wave to meet the sampling theorem.
The high-speed sampling will lead to complexity in signal processing of each sensor signal and significant increase in data processing load. If the sampling interval is extended while keeping the carrier wave at a constant frequency, the signal processing load can be reduced, whereas the frequency of the carrier wave nonconforming to the sampling theorem will make it difficult to adequately extract each sensed signal from the sensor signals and accurately perform the magnetic bearing control such as the levitation control and the motor drive control, and the rotor temperature monitoring.
Moreover, the high-speed sampling requires a device capable of high-speed processing which leads to an increase in cost.
Further, the conventional magnetic bearing system has a problem about instability in a magnetic bearing control due to a temperature rise of a rotor assembly (i.e., rotor, rotary shaft, target member, rotor disc, etc.). FIGS. 19A to 19C are graphs for explaining relationships between a temperature rise of the rotor assembly and respective magnetic bearing control parameters. Each characteristic curve illustrated FIGS. 19A to 19C is one model for simplifying explanation, but not a curve indicative of an actual characteristic.
During operation of a turbo-molecular pump equipped with a magnetic bearing system, a rotor temperature will increase due to heat caused by the friction with evacuation gas, and heat from the motor when it is supplied with a large motor current. In response to an increase in rotor temperature, the rotor has a thermal expansion, and a gap between the rotor assembly and the position sensor is changed in a characteristic curve having a negative inclination, as shown in FIG. 19A. That is, the gap becomes smaller as the rotor temperature becomes higher (i.e., the gap is changed in a direction indicated by the arrow in FIG. 19A).
When the position sensor is an inductance type, it exhibits a characteristic curve having a negative inclination, as shown in FIG. 19B, in response to the change of the gap. That is, an output of the position sensor becomes higher as a distance of the gap becomes smaller (i.e., the output of the position sensor is changed in a direction indicated by the arrow in FIG. 19B). This output of the position sensor corresponds to sensitivity of the position sensor.
As shown in FIG. 19C, if the output of the position sensor becomes higher, a loop gain of the magnetic bearing control circuit is likely to be excessively increased to cause instability in the magnetic bearing control.
Due to the above relationship, the loop gain of the magnetic bearing control circuit becomes higher along with an increase in temperature of the rotor assembly to cause the problem about instability in the magnetic bearing control.
Further, if the vacuum pump is continuously operated even after the rotor is placed in a high-temperature state, resulting negative factors due to thermal expansion, such as permanent deformation and looseness in each portion of the rotor, are likely to cause a problem about occurrence of shaft vibration. This phenomenon becomes prominent along with an increase in a cumulative operation time of the vacuum pump operated when the rotor is in a high-temperature state.